1. Field of the Invention
The invention relates to a magnetic field sensor which uses the magnetooptic effect of magnetic materials. In particular, it provides a magnetic field sensor which can measure a wide range of magnetic fields with a high degree of accuracy.
2. Discussion of Background
In the field of power electronics, the efficient running and automation of electric power systems of increasing capacity necessitates digital control and protection systems for the high voltage plant in substations and the like. Current and voltage sensors for controlling and measuring the current and voltage of high-voltage power transmitters and transformers are indispensable for this purpose. A need exists for the equipment to be small, low in cost and reliable. Current and voltage sensors must be well insulated against high voltages, resistant to electromagnetic faults, miniaturized, and of high performance.
Prior art systems for the control and measurement of current was effected by using a large, wound-type transformer, with a core and windings, to transform the magnetic field produced by the electric current into a current and voltage. However, the windings and core used in these transformers took up a lot of space, and there were problems related with poor insulation against high voltages and poor resistance to electromagnetic noises.
In order to solve the problems associated with the transformers described above, the optical magnetic field sensor has been developed as current sensing devices for electrical systems. In recent years, magnetic garnet single crystal and ZnSe single crystal having a large magnetooptic effect (Faraday Effect) and a little optical absorption have been disclose in reports of optical magnetic field sensors which use these in combination with optical fibers (e.g. National Technical Report, Vol. 29, No. 5, October 1983, pp. 70-80; Keiso, Vol. 26, No. 11, October 1983, pp. 56-60 and Japanese Patent Disclosure No. 58-139082).
These optical magnetic field sensors consist of a light source part (e.g. a light-emitting diode), a magnetic field detection part, containing magnetic garnet single crystal having a Faraday effect (e.g. (Y.Tb).sub.3 Fe.sub.5 O.sub.12, (Y.Sm.Lu.Ca).sub.3 (Fe.Ge).sub.5 O.sub.12, [Y.sub.0.3 Sm.sub.0.5 Lu.sub.1.4 Ca.sub.0.6 Gd.sub.0.2 ](Fe.sub.4.4 Ge.sub.0.6)O.sub.12) a polarizer, a light-measuring part which receives and measures the light which is radiated from the light source part and passes through the magnetic field detection part, and an optical transmission line which links by optical means the light source part, magnetic field detection part and light-measuring part.
As shown in FIG. 7, a magnetic field sensor of this type has a light source 1, a polarizer 2, a Faraday element (magnetic garnet in the form of a thin film) 3 as a magnetooptical element, a polarizer 4 and a photoreceptor element 5, arranged in sequence. They are linked, by optical means for light transmitting such as optical fibres, lenses or the like (not shown). The direction of the axis of easy magnetization of Faraday element 3 is at right angles to the thin film plane, and maze-like magnetic domains are formed in the thin film. This Faraday element 3 has the thin film element disposed at right angles to the direction of the magnetic field to be measured, so that its axis of easy magnetization is parallel to the direction of the magnetic field to be measured (Japanese Patent Disclosure Nos. 58-139082, 58-27071, 58-27072). The light is transmitted at right angles to the thin film plane of Faraday element 3. Polarizers 2 and 4 are arranged so that their axes of polarization make an angle of 45.degree. With respect to each other.
The principle of this optical magnetic field sensor will now be explained. The light radiated from light source 1 first becomes linearly polarized light at polarizer 2, then passes through the Faraday element 3. When the magnetic field applied to Faraday element 3 is 0 (zero), the Faraday element 3 is in a demagnetized state, and since there is no magnetization component in the direction of transmission of the light, the plane of polarization of the light does not rotate. As the axes of polarization between polarizers 2 and 4 are rotated through an angle of 45.degree. with respect to each other, the light from light source 1 is detected by photoreceptor element 5. In this case, the light is reduced only by the amount of attenuation in the members constituting the sensor.
When on the other hand a magnetic field H is applied to Faraday element 3, the magnetic field H induces a magnetization component in Faraday element 3 in the direction of transmission of the light, and the plane of polarization of the linearly polarized light passing through Faraday element 3 is rotated through an angle .theta. proportionate to M, namely EQU .theta.=.theta..sub.F.l.M/M.sub.S
(where .theta..sub.F is the coefficient of Faraday rotation, l is the optical path of Faraday element, M.sub.S is the saturation magnetization). Generally speaking, in magnetic garnet, the magnetization component M changes linearly with the magnetic field until a magnetic field is reached equivalent to the demagnetizing field EQU H.sub.d =N.4.pi.M.sub.s
(where N is the coefficient of demagnetizing field), so the angle of rotation .theta. is also proportionate to H. The larger this angle of rotation .theta. becomes, the greater the change in the intensity P of the light passing through polarizer 4 and detected by the photoreceptor element 5. In the magnetic field sensor described above, therefore, the size of magnetic field H is measured by the change in light intensity P.
However, the arrangement of this kind of magnetic field sensor, in which the direction of the easy axis of magnetization of the magnetic garnet single crystal in the form of a thin film is at right angles to the thin film plane, maze-like magnetic domains are formed in the film plane, the axis of easy magnetization of the magnetic garnet single crystal is parallel to the direction of the magnetic field to be measured, and the magnetic field is measured by the changes in magnetization produced by movements of the walls of the domains, gives rise to the following problems. If the diameter of the beam of light is small, then problems will occur due to the nonuniformity of the pattern width of the magnetic domains, and by the width of these domains. This means that if the magnetic field to be measured changes dynamically, the angle .theta. may also change, depending on the shape and direction of the magnetic garnet single crystals, in relation to the same magnetic field being measured. This makes it difficult to measure the magnetic field with a high degree of accuracy. A further problem is that since, if the saturation magnetization is large, the energy of the demagnetizing field is also increased, the saturation magnetization can not, in general, be permitted to be large, with the result that it is difficult to measure a high magnetic field.
Non-magnetic ZnSe can be used in place of magnetic garnet single crystal when measuring a high magnetic field. But there is a problem here too, in that ZnSe has only a small magnetooptic effect and low sensitivity.
In addition to the above mentioned problems that are liable to be caused by the width of the magnetic domains, the conventional optical field sensors have a limitation in measuring magnetic fields, due to limitations with respect to saturation magnetization Ms. This occurs, because the measurement sensitivity S with respect to M.sub.s is expressed in the case of the magnetized direction or the shape of the magnetic garnet single crystals to be used as Faraday elements as; EQU S=.theta..sub.F.l/M.sub.s.